1. Field of the Invention
The present invention relates to a power supply device for a copying machine, LBP, BJ printer, and FAX and, more particularly, to a voltage resonance power supply device.
2. Related Background Art
A conventional voltage resonance power supply has been proposed by Japanese Laid-Open Patent Application No. 5-130776. FIG. 14 shows the circuit arrangement of this power supply, and FIGS. 15A to 15D show the operation waveform of a transistor 3 in FIG. 14 and the waveform of an electrical current supplied to a diode 8 on the secondary side.
In FIG. 14, the power supply device comprises an input power supply 1, a resonance capacitor 2, a transistor 3, a transformer 4 having primary and secondary windings 5 and 6, a gate drive winding 7 for the transistor 3, a diode 8, an output capacitor 9 for the transformer 4, an activation resistor 10, an ON width determination circuit 11, a feedback circuit 12, a gate-direction electrical current switching circuit 13, and a capacitor 17.
FIG. 15A shows the waveform of a drain voltage Vds of the transistor 3, FIG. 15B shows the waveform of a drain electrical current Id of the transistor 3, FIG. 15C shows the waveform of an electrical current 12 which flows in the rectification diode 8 on the secondary side, and FIG. 15D shows the drawn voltage Vds and drain electrical current Id of the transistor 3 upon switching the transistor 3 from ON to OFF, while being enlarged along the time axis.
The circuit shown in FIG. 14 corresponds to a self-excited switching flyback converter, and operates basically in the same way as a so-called RCC. More specifically, the activation resistor 10 temporarily turns on the transistor 3 to activate the circuit. When the transistor 3 is ON, an input voltage is applied to the primary winding 5 of the transformer 4, and a proportional voltage is induced in the drive winding 7. That voltage is input to the gate-direction electrical current switching circuit 13, the F terminal of which detects zero drain potential of the transistor 3. Then, the circuit 13 is turned on from its H terminal to G terminal to maintain the transistor 3 ON via the capacitor 17. At this time, the drain electrical current Id linearly increases, as shown in FIG. 15B.
The feedback circuit 12 sends a signal to the ON width determination circuit 11 in accordance with the output voltage. The circuit 11 determines the ON width and turns off the transistor 3. When the transistor 3 is turned off, the drain voltage of the transistor 3 immediately rises due to energy built up on the capacitor owing to the voltage resonance effect of the resonance capacitor 2 and primary winding 5, and magnetic energy supplied by the primary winding, and the diode 8 on the secondary side is enabled eventually, thus maintaining the drain voltage below a predetermined value. As the secondary electrical current, a triangular wave electrical current flows, as shown in FIG. 15C, and excitation energy is radiated toward the secondary side. After the energy radiation, the drain voltage starts a resonance damped oscillation by the energy built up on the capacitor, and falls relatively slowly. The drain voltage becomes zero eventually. When the drain voltage has become zero, the gate direction electrical current switching circuit 13 repeats the above-mentioned operations.
However, in the above prior art, when the drain voltage of the transistor 3 becomes zero by its resonance damped oscillation, the transistor 3 is turned on to enable zero voltage switching, thereby reducing switching losses. However, as shown in FIG. 15D, when the transistor 3 is turned off, the drain voltage changes abruptly, resulting in an increase in switching losses due to superposition of the drain voltage and electrical current, and in increased noise. As shown in FIG. 15C, a triangular wave electrical current flows in the rectification diode on the secondary side, and switching losses and noise are produced in the rectification diode due to abrupt changes in electrical current.